This invention relates generally to electromagnetic clutches, such as for use in controlling the transmission of an automobile engine output to a refrigerant compressor in an automobile air-conditioning refrigerating system, and, in particular to an improved amature structure for an electromagnetic clutch.
U.S. Pat. Nos. 3,044,594 and 3,082,933 disclose electromagnetic clutches which are used between an automobile engine and the compressor. The shown electromagnetic clutch dislcosed in these patents comprises a pulley mounted on a bearing which is, in turn, mounted on a tubular extension of a compressor housing and which is rotated by a belt from the automobile engine. The pulley is provided with an axial end plate of a magnetic material in which one or more concentric arcuate or annular slits are formed to define a plurality of concentric annular magnetic pole pieces. A drive shaft of the compressor extends through the tubular extension. A hub is fixed to the extending terminal end of the drive shaft, and an annular armature plate is joined by leaf springs to the hub in such a fashion that the armature plate faces the annular concentric pole pieces with an axial air gap therebetween. A magnetic coil is mounted on the tubular extension to supply magnetic flux for attraction of the armature plate to the magnetic pole pieces.
Thus, when the magnetic coil is energized, the drive shaft is rotated together with the pulley by the engine output, and when the magnetic coil is not energized, the pulley is rotated by the engine but the compressor is not driven.
In the conventional electromagnetic clutch, one or more annular grooves are formed in the surface of the armature opposite to the magnatic pole pieces corresponding to one or more concentric circles on which the slits are disposed, in order to strengthen the magnetic attraction force between the magnetic pole pieces and the armature.
However the formation of grooves in the armature surface increases the magnetic resistance of the armature, which causes a reduction in the magnetic attraction force between the armature plate and the magnetic pole pieces. Therefore, the armature must be made of thicker magnetic plate in order to secure the advantage of the formation of the grooves, thereby increasing the weight of the armature plate and, therefore, the electromagnetic clutch.
Moreover, leaf springs for joining the armature plate to the hub are conventionally fixed to the armature plate by rivets, which requires the formation of holes in the armature plate. In addition, a considerable amount of time is required for the fixing operation by the use of rivets.
Furthermore, in fixing the leaf springs to the hub, washers are conventionally used between the leaf springs and the hub to prevent the leaf springs from coming into contact with the peripheral edge of the hub at a time when the armature is attracted to the magnetic pole pieces of the pulley. But the use of washers complicates the fixing of the leaf springs to the hub.